grav-admin-first/user/plugins/admin/themes/grav/templates/partials/userinfo-avatar.html.twig

{% if data.avatar %}
    <label><img src="{{ data.getAvatarUrl() }}" /></label>
{% else %}
    <label><img referrerpolicy="no-referrer" src="https://www.gravatar.com/avatar/{{ data.email|md5 }}?s=200" /></label>
{% endif %}